Machine-to-machine (M2M) communication (also referred to as machine-type communication (MTC)) is used to establish communication between devices and between devices and humans. The communication may include, for example, an exchange of data, signaling, measurement data, configuration information, and any other suitable types of communication. The device size may vary from that of a wallet to that of a base station. M2M devices are often used for applications such as sensing environmental conditions (e.g., a temperature reading), metering or measurement (e.g., electricity usage, etc.), fault finding or error detection, among other applications. In these applications, M2M devices are active very seldom but over a consecutive duration depending upon the type of service (e.g., about 200 ms once every 2 seconds, about 500 ms every 60 minutes, etc.). An M2M device may also perform measurements on other frequencies or other Radio Access Technologies (RATs).
An MTC device is expected to be of low cost and/or low complexity. In some cases, aa MTC or M2M device may be interchangeably referred to as a low-cost and/or low-complexity user equipment (UE) or a low-cost and/or low-complexity wireless device. The low-complexity UE envisaged for M2M operation may implement one or more low-cost features such as smaller downlink (DL) and uplink (UL) maximum transport block size (e.g., 1,000 bits) and/or reduced DL channel bandwidth of, for example, 1.4 MHz for the data channel (e.g., Physical Downlink Shared Channel (PDSCH)). Further possible features of a low-cost UE include a half-duplex Frequency Division Duplex (HD-FDD) and one or more of the following features: a single receiver (1 Rx) at the UE; smaller DL and/or UL maximum transport block size (e.g., 1,000 bits); and reduced UL and DL channel bandwidth of 1.4 MHz for the data channel. A low-cost UE may be considered a class of low-complexity UE. A low-complexity UE (e.g., a UE with 1 Rx) may be capable of supporting enhanced coverage mode of operation.
The path loss between an M2M device and the base station can be very large in some scenarios, such as when the M2M device is used as a sensor or metering device located in a remote location (such as in the basement of a building). In these scenarios, the path loss can be worse than 20 dB compared to normal operation, making reception of signals from the base station challenging. In order to address these issues, the coverage in UL and/or in DL has to be substantially enhanced with respect to normal coverage (also referred to as legacy coverage). In some cases, this is realized by employing one or a plurality of advanced techniques in the UE and/or in the radio network node for enhancing the coverage. Some non-limiting examples of these advanced techniques include, but are not limited to: transmit power boosting, repetition of transmitted signal, applying additional redundancy to the transmitted signal, and using an advanced and/or enhanced receiver. When employing such coverage enhancing techniques, the M2M devices are generally regarded as operating in “coverage enhancing mode.” A low-complexity UE (e.g., a UE with 1 Rx) may also be capable of supporting enhanced coverage mode of operation.
Radio link monitoring (RLM) is performed to monitor the radio link quality of the connected serving cell and use that information to decide whether a UE is in-sync or out-of-sync with respect to that serving cell. RLM is carried out by a UE performing one or more measurements on DL reference symbols (e.g., Cell-Specific Reference Signals (CRS)) in RRC_CONNECTED state. If the results of RLM indicate a number of consecutive out-of-sync indications, then the network may declare radio link failure (RLF) until the monitoring indicates several consecutive in-sync indications. The actual procedure may be carried out by comparing the estimated DL reference symbol measurements to some target block error rate (BLER), Qout and Qin. Qout and Qin correspond to the BLER of hypothetical Physical Downlink Control Channel (PDCCH)/Physical Control Format Indicator Channel (PCFICH) transmissions from the serving cell.
Radio measurements done by the UE are typically performed on the serving cell as well as on neighbour cells over some known reference symbols or pilot sequences. The measurements may be done on cells on an intra-frequency carrier, inter-frequency carrier(s), as well as on inter-RAT carriers(s) (depending upon the UE capability (i.e., whether it supports that RAT)). To enable inter-frequency and inter-RAT measurements for UE requiring gaps, the network has to configure the measurement gaps.
The measurements may be done for a variety of purposes. Some example measurement purposes include, but are not limited to: mobility; positioning; self-organizing network (SON); minimization of drive tests (MDT); operation and maintenance (O&M); network planning and optimization, and other purposes. Examples of measurements in LTE include, but are not limited to: Cell Identification (also referred to as Physical Cell ID (PCI) acquisition); Reference Symbol Received Power (RSRP); Reference Symbol Received Quality (RSRQ); acquisition of system information (SI); cell global ID (CGI) acquisition; Reference Signal Time Difference (RSTD); UE receive-transmit (RX-TX) time difference measurement; and RLM (which consists of Out-of-Synchronization (out-of-sync) detection and In-Synchronization (in-sync) detection). Channel State Information (CSI) measurements performed by the UE are used for scheduling, link adaptation, and other operations by the network. Examples of CSI measurements or CSI reports include Channel Quality Indicators (CQI), Precoding Matrix Indicator (PMI), Rank Indicator (RI), etc. They may be performed on reference signals like CRS, Channel State Information Reference Signals (CSI-RS), or Demodulation Reference Signals (DMRS).
The measurements may be unidirectional (e.g., DL or UL) or bidirectional (e.g., having UL and DL components such as Rx-Tx, Round Trip Time (RTT), etc.).
The DL subframe #0 and subframe #5 carry synchronization signals (i.e., both Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS)). In order to identify an unknown cell (e.g., a new neighbor cell), a UE has to acquire the timing of that cell and eventually the PCI. This is referred to as cell search, cell identification, or even cell detection. Subsequently, the UE also measures RSRP and/or RSRQ of the newly identified cell in order to use itself and/or report the measurement to the network node. In total, there are 504 PCIs. The cell search is also a type of measurement. The measurements are done in all Radio Resource Control (RRC) states (i.e., in RRC idle and connected states).